</head>   
<body>   
<h1>Ajax动态显示时间</h1>   
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />  
<input type="button" value="开始显示时间" id="go" onclick="start()" />   
<p>当前时间：<font color="red"><span id="showtime"></span></font></p>   
</body>   
<script type="text/javascript">  
var xmlHttp;  
function createXMLHttpRequest(){  
 if(window.ActiveXObject){  
  xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");  
 }  
 else if(window.XMLHttpRequest){  
  xmlHttp = new XMLHttpRequest();  
 }  
}  
function start(){  
 createXMLHttpRequest();  
 var url="getTime.php";  
 xmlHttp.open("GET",url,true);  
 xmlHttp.onreadystatechange = callback;  
 xmlHttp.send(null);  
}  
function callback(){  
 if(xmlHttp.readyState == 4){  
  if(xmlHttp.status == 200){  
   document.getElementById("showtime").innerHTML = xmlHttp.responseText;  
   setTimeout("start()",500);  
  }  
 }  
}  
</script>  
</html>  